Power amplifier capable of simultaneous operation in two classes

ABSTRACT

An electronic amplifying apparatus intended for sound reproduction and for music instrument amplification wherein two parallel amplifiers, working simultaneously, each in a different class of operation, are fed from a single driver or signal source and whose outputs are combined. In the preferred embodiment at least two pairs of push-pull vacuum tubes are required and they are arranged so that one pair operates Triode Class A while the other pair (or pairs) operates Pentode Class AB (or Class B). The circuit can then be optimized so that the desirable sonic characteristics of Class A Triode operation are imparted into the Class AB (or Class B) Pentodes which actually produce all or nearly all of the power. The output power waveform has the high power and efficiency typical of Class AB (or Class B) Pentode operation but without the detrimental sonic side effects, namely, there is the complete absence of crossover or &#34;notch&#34; distortion, and there is a &#34;soft&#34; gradual onset of clip. Both of these traits are highly desirable and characteristic of the notoriously inefficient Class A Triode operation. 
     On/Off switching of various pairs of output tubes may further be employed to add flexibility to the system with regard to power availability and sonic performance. Single ended configurations as well as solid state devices or hybrid combinations could be arranged to operate in simultaneous different classes of operation and enjoy its attendent benefits.

This is a division of Ser. No. 489,915, filed Apr. 29, 1983, and nowU.S. Pat. No. 4,532,476, which is a continuation of Ser. No. 278,717,filed June 29, 1981, and now abandoned.

FIELD OF THE INVENTION

High fidelity audio power amplification for sound reproduction and formusical instruments, principally electric instruments, e.g. electricguitar, electric bass, electric piano, synthesizer, and the like.

BACKGROUND OF THE INVENTION

Class A Triode operation is well known to discriminating audiophilesbecause of its characteristic musicality and "warmth". This warmth oftonality can be ascribed to the avoidance of crossover distortion(because in a push-pull configuration neither device ever approachescut-off) and to the soft or gradual onset of clip when dynamic levelsexceed available undistorted power capabiity. Unfortunately thesevirtues of Triode Class A operation carry with them severe penalities ofeconomy, efficiency and low power capability. Operating Class A meansthat less than half the supply power available can be converted touseful work, and that the devices themselves must be de-rated due tohigh zero-signal current draw. Connecting the screen grid of a Pentodeto its plate, causing it to operate as a Triode, further reducespotential power gain by about half.

These penalties of price and power availability (not to mention heat)have prevented Class A Triode operation from ever gaining popularity inthe Music Instrument field even more than in sound reproductionapplications. This is particularly unfortunate for a couple of reasons:First, due to the extreme dynamic levels produced by the plucked string(e.g., electric guitar), amplifier output clip is virtually unavoidable.Second, musicians have done more than learn to live with amplifierdistortion, they have incorporated it creatively into their musicalexpression and have become connoisseurs of different distortioncharacteristics. In the "heavy metal" idiom as well as the blues andpopular milieu, elements of amplifier distortion are so strongly part ofthe guitar sound as to often comprise about 50 percent of actual soundoutput. Further, circuits to generate "desirable" distortion effects arewell known and in use. These can be used to simulate output circuitdistortion independent of output power level. If the history ofprofessional choice were used as a yardstick of "desirable distortion"then clearly the circuit of Smith (U.S. Pat. No. 4,211,893) is a firstchoice. Here the option of distortion is available via remote switchingand can be controlled to enhance solo or "lead" playing. It isinteresting to note that Smith's circuit, which generates the distortionin the pre-amplifier section, uses Class A Triodes throughout.

The present invention, however, deals with output power amplificationand the inherent distortion characteristics of different circuitconfigurations. Music is by nature a series of transient and fleetingevents. The attack of any given musical note is of particular concern tothe musician (as well as the listener) and much of a player's learnedtechnique and expression revolves around the attack of the note. Such isclearly the case with all stringed instruments--including piano--as wellas reed, brass and percussion instruments. Further, it can be shown thatthe manner in which an audio amplifier handles these transient attacksis the single most important factor that distinguishes an outstandingamplifier from one which is merely acceptable in both reproduction andlive performance applications. The steady state distortioncharacteristic of most modern amplifiers is excellent: distortion of anytype cannot be heard and competitive measurements have become pointless.Unfortunately though, there is no standard measurement of rating fordistortion performance under actual dynamic conditions of use. Toproduce realistic sound levels in one's living room of a symphony orpiano, without some clipping of the transient peaks would require anamplifier of at least 1000 watts and preferably much more.

So the demand for high power is obvious to the home user as well as themusician, yet the occurrance of amplifier distortion--clipping ofpeaks--remains an unavoidable fact of life to both. Whereas certaintypes of amplifier circuitry produce high power efficiently, a highsonic penalty is paid: their distortion is noticeable, harsh anddisturbing. This is the typical "odd order" harmonic distortion whichoccurs in Class B type amplifiers and is the product of hard clip andsharp current transfer from push to pull, causing "crossover" or "notch"distortion. On the other hand, the Triode Class A circuit produces nocrossover distortion and has a "soft" clip. The sound of amplifierdistortion at clip and beyond is almost unnoticeable because clip doesnot occur suddenly, and when it does, it is characterized by thepredominance of even order products which are actually harmoniousmusically (that is consonant, not dissonant) to the fundamental. But thepenalty is power. Such a circuit is expensive and inefficient.

The well known amplifier Class AB offers some little improvement.Although the output devices operate Class A at low power, they becomemore and more Class B when driven harder and a somewhat harsh soundingdistortion with an abrupt onset and visible crossover occurs at thecrucial time: at clip. The power output and efficiency with Pentodes inan AB arrangement is fairly high however.

BRIEF DESCRIPTION OF THE INVENTION

In my improved amplifier circuit, benefits are derived continuously fromboth classes because the amplifier actually operates simultaneously inboth Triode Class A and Pentode Class AB (or B). The Figure shows fouroutput devices in a push-pull parallel configuration. Electron vacuumtubes are shown but the principles underlying my invention ofsimultaneous operation in two classes also apply to solid state devicesof all types. A pair of Pentode tubes (V5 & V6) are configured asTriodes in a push-pull arrangement. They are biased in such a way thatthey run fully Class A, that is, they draw substantial plate current atall times throughout the duty cycle and never approach cutoff. At thesame time a second pair of output devices may be switched on in parallelto the Class A Triodes for simultaneous different classes of operationwhen higher output power is called for. This second pair of tubes (V7 &V8) operates as push-pull Pentodes Class AB (or B). This means that thefixed bias, being substantially greater than that for the Triode pair,allows use of much higher Plate voltage while still observing safelevels of dissiptation. Further, the fact that these devices arePentodes, renders the available power gain even greater. Signal driveand grid bias for the two pairs are disturbed through a divider networkadjusted for optimum balance. Such a balance can be achieved so that thenet composite operation retains the virtues of both types of device(Triode and Pentode) and of both classes of operation (Class A and ClassAB). Since abundant current is always flowing through both halves of theoutput transformer to supply the Class A Triodes, the smoothness oftheir transfer characteristic effects and smooths the apparent transfercharacteristic through the transformer of the AB Pentode pair also. Evenat maximum power no crossover distortion is evident. Furthermore, theTriode factor contributes the characteristics of soft, gradual clipwhich can be made to occur in advance of and to predominate over theclip of the Pentodes at all power levels.

So the circuit enjoys the benefits of high power and efficiencycontributed by the Pentodes running Class AB (or even Class B) but hasthe preferred distortion characeristics--clip and crossover--of Triodesrun Class A. Usable tube life is also extended in an amplifier operatingsimultaneously in different classes because the Class AB Pentodes can bebiased very high--into Class B--for cool low current, high poweroperation, while deterioration of the Class A Triodes results in verylittle sonic degradation since their power contribution is small.

Part of the present disclosure is the preferred driver amplifiercircuitry which maximizes the sonic benefits of simultaneous differentclasses of operation. A pair of Class A Triode input devices serves thetwofold purposes of amplifying the incoming signal and splitting it intotwo phase inverted components. Excellent linearity and accurate phaseinversion are accomplished by using a differentiating amplifier pairwhose cathodes are biased through a constant current device. The firsttriode (V1) operates conventionally with its grid serving as the inputelement and its plate furnishing the output. The second triode (V2)derives its signal input from its shared cathode configuration with V1.The grid of V2 is grounded (or held slightly above ground to allow theinjection of negative feedback) and amplified phase inverted output(with respect to output at the plate of V1) is present at V2's plate.The use of a constant current source in the common cathode circuitgreatly helps to insure linearity of V1-V2 over a wide range ofoperating conditions. A user operable switch means is provided to allowselection of full loop feedback, local feedback only, or no feedback atall. Even though judicious applications of negative feedback arewonderously effective in producing impressive distortion specificationsunder static testing conditions, many listeners will prefer the greaterdynamic realism and open, unrepressed impact of less--or ultimatelyno--negative feedback. A dual buffer amplifier (V3 & V4) is used toprovide sufficient drive amplitude for the amplifier output states.Tubes V3 and V4 are, as usual, operated as Class A Triodes but theircommon cathodes are carefully biased to maximize one of the effects ofsimultaneous operation in different classes: soft and gradual clipping.In other words, the plate loading and cathode biasing of V3-V4 iscarefully configured so that the onset of clip at V3-V4 is soft andoccurs slightly before clip occurs in the amplifier output stages.

A novel configuration with regard to all coupling capacitors is usedthroughout the driver/buffer circuit. Capacitors with large values areoften used in audio preamplifier circuits to allow linear frequencyresponse down to 20 hz or below. Usually the circuit designer (who isaware of the negative consequences of large capacitor values) must makea compromise based on three trade-offs. If small value capacitors areused, low frequency linearity can not be obtained. If large valuecapacitors are used, phase shift (or time delay) increases withincreasing capacitor value and improved low frequency linearity becauseeach AC signal wave must charge the capacitor against its time constantwith the attendent circuit resistance. This problem is remedied inconventional practice by keeping a fairly high impedance (or resistance)on the output side of the capacitor. The undesired (and overlooked)consequence of this solution in a low feedback amplifier is that thezero DC reference state of the capacitor output becomes unstable andgyrates with strong signal fluctuations. This is apparently overlookedbecause most circuit designers concentrate on the steady stateperformance of their amplifiers where the DC gyrations on bigtransients, which settle out after about one second, are usually notdiscovered or dealt with. In my improved driver/buffer circuit thesethree problem/trade-offs are virtually eliminated by using largecapacitors with a fairly low impedance across their inputs and a veryhigh impedance at their outputs.

So then, to summarize, some novelties about my amplifier are:

1. A differential amplifier has a constant current cathode source andacts as phase inverter/first amplifier followed by 2; a dual bufferamplifier whose loading and biasing are arranged to produce clip in acontrolled fashion in harmony with 3; a pair of parallel output poweramplifiers working simultaneously and in differing classes of operationwhose outputs are combined. 4. Negative feedback may be appliedselectively: full loop, local or avoided entirely. 5. To provide lowfrequency linearity while still ensuring faithful AC and DC response inthe time domain, large coupling capacitors are arranged throughout insuch a fashion that the impedance at their inputs is fairly low whilethe impedance at their outputs is very high.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows a schematic diagram of an amplifier circuit inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 the input 1 is connected to the grid 3 of V1 via conductor 2.Grid leak resistor 14 is connected to ground 25. The cathode 5 of V1 istied directly to the cathode 6 of V2 and both are energized from anegative B- source 12. Use of a constant-current source device 11 in thecathode circuit accomplishes vastly improved linearity of V1 and V2under widely ranging operating conditions. As V1 amplifies, signalfluctuations which appear on its cathode 5 also appear on the cathode 6of V2 and are used to drive V2. The grid 4 of V2 is for all intents andpurposes grounded although a small resistor 13 may be used to allow theinjection of negative feedback to the grid 4 via an RC network 15, 16. Aswitch means 91 is then provided to select negative feedback originatinglocally from the output of buffer stage V3 through a blocking capacitor96 and a series resistor 92. Or as alternatives to suit programmaterial, choice of speaker and listener preference, switch 91 can alsobe used to select overall negative feedback, which signal would derivefrom a point 95 at the amplifier output and be buffered through resistor93. As the third alternate, the switch 91 offers the user the selectionof no negative feedback whatever, which listening tests demonstrate ismost often preferred. Plates 7, 8 of triodes V1 and V2 are connected tothe high voltage B+ supply 80 through high impedance load resistors 9and 10. Amplified, split phase signals are conducted from the plates 7,8 of V1 and V2 to the grids 29, 30 of V3 and V4 respectively viacoupling capacitors 19 and 20 which are large in value. Grid leadresistors 23, 24 represent a very high impedance while much lowerimpedance resistors 21, 22 provide DC stability to the large capacitors19, 20 in the very high impedance grid circuit of V3 and V4 inclusive.Cathodes 27, 28 of V3 and V4 respectively are energized through resistor26 whose value is selected in conjunction with the values of loadingresistors 35, 36 to cause a soft and gradual clip of V3, V4 to occurjust prior to the onset of clip in the Class A Triode section of theamplifier output state. The plates 31, 32 of V3 and V4 are energizedfrom the B+ high voltage supply 80 through resistors 33, 34. Output fromthe plates 31, 32 of V3 and V4 is coupled to the output stages throughcapacitors 37 and 38, both of which have a large value, and resistors 83and 84 which are used to present a very high impedance to the output ofcapacitors 37 and 38. Negative bias for the output stage comprised ofV5, V6, V7 and V8 derives from a supply represented at 71. This negativeDC bias flows through an output tube balancing network comprised ofpotentiometer 69 and drain resistors 81 and 82. Bias for the Pentodes V7and V8 is obtained through low value resistors 43 and 44, and ismaintained at a level that provides Class AB (or Class B) operation forthese Pentodes. Screen grids 61, 62 of these Pentodes (V7, V8) are fedfrom a high voltage supply 74 through low value resistors 63 and 64.Pentode plates 59, 60 are impedance matched to the full primary winding72 of the output transformer with one plate at each of the ends 87, 88.The cathodes 67, 68 of V7 and V8 are selectively energized from ground25 through a user operable switch means 70. When the switch is open, nocurrent flows, V7 and V8 are effectively in a "standby" mode and theamplifier comprises functionally of the pair of Class A Triodes V5 andV6. With the switch 70 closed, the amplifier circuit comprising V7 andV8 (and any other number of Pentode AB output pairs in parallel) andtheir inclusive parts becomes operational and in fact contribute thevast majority of power to the common output 75 as the plates 59 and 60of V7 and V8 are connected to the output transformer primary 72 throughconductors 76 and 77. Pentodes shown as V5 and V6 are made to functionas Triodes because their plates 45, 46 are tied to their respectivescreen grids 51, 52 through low value resistors 53, 54. Cathodes 57, 58are energized from ground 25. Grid bias on V5 and V6 is reduced to a lowvalue compared to the AB Pentode pair by the divider networks comprisingresistors 39 and 41 for V5 and resistors 40 and 42 for V6. Furthermore,the values selected for these divider networks and other resistors inthe bias network--as well as in the signal path--are chosen to cause theonset of clip in the Triode Class A pair to occur slightly before thePentode AB pair(s) begins to clip. Taps 89, 90 can be provided at lowerimpedance points on the output transformer primary 72 to properly matchthe plate loads imposed by the Class A Triode pair. Voltage droppingresistors 47 and 48 can be used to reduce dissipation of the plates 45,46 of the Class A Triodes V5 and V6 to safe levels while still allowingthe maintenance of full DC voltage on the Pentode pair V7, V8. Bypasscapacitors 49, 50 may be used to restore signal gain lost through thedropping resistors 47, 48.

Though the invention has been described with respect to a specificpreferred embodiment thereof, many variations and modifications willimmediately become apparent to those skilled in the art. It is thereforethe intention that the appended claims be interpreted as broadly aspossible in view of the prior art to include all such variations andmodifications.

I claim:
 1. An amplifier, primarily for audio power musical instrumentsand sound reproduction systems including:(a) at least two pairs ofelectron discharge devices, each device having an input elecrtrode andan output electrode, each said pair of electron discharge devices beingarranged in a push-pull configuration; (b) means connecting each of saidoutput electrodes to a common utilization device; (c) first inputimpedance means coupled to one said pair of electron discharge devicesfor causing said one pair of electron discharge devices to operate inClass A; and (d) second input impedance means having a lower impedancethan said first input impedance means coupled to the other said pair ofelectron discharge devices to cause said other pair of electrondischarge devices to operate in Class AB, (e) said first means andsecond means including means arranged to cause continuous operation ofsaid one pair of electron discharge devices in Class A when said otherpair of electron discharge devices is driven into saturation.
 2. Anamplifier as set forth in claim 1 wherein said utilization device is atransformer, the magnetic field in the entire said transformer beingmaintained when said other said pair of electron discharge devices iscut off.
 3. An amplifier according to claim 2 including switch means forselectively rendering at least one electron discharge device pairinoperative.
 4. An amplifier as set forth in claim 3 wherein the inputelectrodes of said one pair of electron discharge devices have a fixedbias thereon substantially less than the fixed bias on the inputelectrodes of said other pair of electron discharge devices.
 5. Anamplifier according to claim 2 wherein the electron discharge devicesare vacuum tubes.
 6. An amplifier as set forth in claim 5 wherein theinput electrodes of said one pair of electron discharge devices have afixed bias thereon substantially less than the fixed bias on the inputelectrodes of said other pair of electron discharge devices.
 7. Anamplifier as set forth in claim 2 wherein the input electrodes of saidone pair of electron discharge devices have a fixed bias thereonsubstantially less than the fixed bias on the input electrodes of saidother pair of electron discharge devices.
 8. An amplifier according toclaim 1 including switch means for selectively rendering at least oneelectron discharge device pair inoperative.
 9. An amplifier as set forthin claim 8 wherein the input electrodes of said one pair of electrondischarge devices have a fixed bias thereon substantially less than thefixed bias on the input electrodes of said other pair of electrondischarge devices.
 10. An amplifier according to claim 1 wherein theelectron discharge devices are vacuum tubes.
 11. An amplifier as setforth in claim 10 wherein the input electrodes of said one pair ofelectron discharge devices have a fixed bias thereon substantially lessthan the fixed bias on the input electrodes of said other pair ofelectron discharge devices.
 12. An amplifier as set forth in claim 1wherein the input electrodes of said one pair of electron dischargedevices have a fixed bias thereon substantially less than the fixed biason the input electrodes of said other pair of electron dischargedevices.